BOSTON — Common wisdom in astronomy says that once a planet has collected about 10 Earths’ worth of rock, it becomes a gas giant like Neptune or Saturn. The exoplanet Kepler-10c didn’t get that memo. With 17 times the mass of Earth, the distant planet is the heaviest rocky planet known. And astronomers have no idea how it formed.

With the mass of Neptune squeezed into a ball only about 2.5 times as wide as our planet, gravity on Kepler-10c is three times stronger than Earth’s, astronomer David Latham reported June 2 at the American Astronomical Society meeting. “I wouldn’t want to be a giraffe on this planet,” said Latham, of the Harvard-Smithsonian Center for Astrophysics.

The planet is one of two orbiting Kepler-10, a sunlike star 564 light-years away in the constellation Draco. Both planets are toasty-warm and close to their star: Kepler-10c whips around the star once every 45 days, while 10b’s period is even shorter. Both planets were among the first discovered by the Kepler space telescope; 10b was, in fact, Kepler’s first confirmed rocky planet.

Astronomers have known of 10c’s diameter and short period for nearly three years, but they only recently measured its mass — and thereby calculated its density and realized that the planet is made of rock. Researchers measured Kepler-10c’s mass by tracking how much its star gets yanked around by the planet’s gravity.

At a press conference, Dimitar Sasselov, another Harvard astronomer, said the research team expected the planet to weigh about the same as two Earths, based on a planet around another star with a similar diameter and period. A mass of 17 Earths was a surprise. The researchers reported the mass at the conference and in a paper posted online May 30 at arXiv.org.

Kepler-10c appears to be the first of a new class of exoplanet: a “mega-Earth.” “Every time we get complacent, we find something new,” said Sara Seager, an MIT astrophysicist. Astronomers thought that rocky planets maxed out at around 10 Earth masses, she explained. Beyond that mass, growing planet embryos would quickly suck down hydrogen and helium from the disk of raw material encircling their infant sun. These lightweight gases can resist compression from gravity, which results in planets with thick, fluffy atmospheres.

But Kepler-10c breaks those rules. “When one type of planet is found, that’s usually the tip of the iceberg,” Seager said. “There are probably many, many more of them.”

A rocky planet that surpasses 10 Earth masses will most likely keep theorists busy as they figure out how to explain its origin. “I’m more than happy to accept that I was wrong about this,” Sasselov said. Theories about planet building are sometimes shots in the dark to help interpret data, he said. But ultimately, observations are the final arbiter. “We’re discovering new populations of planets that hadn’t been thought of before,” he noted.

Kepler-10c is also odd for its age. Its star, and presumably the planet, is about 11 billion years old — almost as old as the Milky Way. (By contrast, the sun is only 4.6 billion years old.) Astronomers think that old stars are less likely to host rocky planets. Ancient stars, the theory goes, formed when the heavier elements that make up rocky planets — carbon, oxygen and silicon — were much less prevalent than they are today. “We don’t know how to make this planet,” Sasselov said.

He added that Kepler-10c may expand the hunt for alien life. Astrobiologists had mostly ruled out older stars as places to look because rocky planets would be rare. But if an 11 billion-year-old sun can host the most massive rocky planet known, then the number of potentially habitable environments around other stars just got bigger.